to the previously mentioned genetic alterations in the apoptotic machinery, which can include p53 mutations, upregulation of Bcl-2 and cFLIP, and downregulation of Bax. The same mutations may also account for resistance to TRAIL-mediated apoptosis acquired by some colon carcinoma cell lines, despite even higher levels of TRAIL receptors in tumor cells as compared with normal colonic epithelium.

5. LIVER

The liver is a complex and exceptionally specialized organ with metabolic, synthetic, and detoxifying functions. This complexity is reflected in its cellular composition, which includes several cell populations: (1) hepatocytes, multifunctional epithelial cells representing the vast majority of liver parenchyma; (2) hepatic stellate cells (HSC), located in the space of Disse and involved, among other functions, in the fibrogenic process; (3) sinusoidal endothelial cells, which form a fenestrated endothelium that lines the liver sinusoids and allow direct contact between hepatocytes and plasma;

(4) Kupffer cells (KCs), liver macrophages located within the lumen of the liver sinusoids and involved in the liver’s response to toxins, infections, and other stresses; (5) biliary epithelial cells or cholangiocytes, which line the lumen of the biliary tree and participate in bile formation. For its unique nature and anatomical location, the liver is exposed to a multitude of toxins and infectious agents coming from the gastrointestinal tract through the portal blood flow and therefore is highly susceptible to tissue injury. To protect itself, the liver has evolutionarily developed several adaptive responses, including an exquisite sensitivity to apoptosis to promptly eliminate damaged or infected cells and a unique ability to regenerate up to 70% of its tissue to counterbalance the cell loss. However, when the cellular loss exceeds the liver regenerative capacity, the organ is no longer able to fulfill its functions, and hepatic failure occurs. In many acute and chronic liver diseases, such as viral and autoimmune hepatitis and cholestatic disease; after chronic exposure to toxins, drug, or alcohol; or in transplantationassociated liver damage, including ischemia-reperfusion injury and graft rejection, regeneration may not fully compensate the tissue loss caused by excessive apoptosis. Moreover, engulfment of greater amounts of apoptotic bodies by KCs may, in these conditions, exacerbate liver inflammation and tissue damage by amplifying death receptor–mediated hepatocyte apoptosis through production of FasL and TNF-α by the KCs themselves. As a result, functional hepatic parenchyma is gradually replaced by fibrotic scar tissue synthesized largely

by activated HSCs, eventually preventing the liver from functioning properly, a condition referred to as liver fibrosis or, in its end-stage, liver cirrhosis. The scar tissue blocks the flow of blood through the organ and drastically slows its functions until hepatic failure occurs. Therefore, excessive hepatocyte apoptosis promotes the development of hepatic fibrosis. Conversely, as activated HSCs are the main source of type I collagen, the principal matrix protein responsible for the development of fibrosis, a therapeutic approach aimed to selectively eliminate them could potentially be beneficial to attenuate liver fibrosis.

Several stimuli can induce apoptosis in the liver, but its cells are particularly sensitive to death receptor– mediated apoptosis as a result of the relatively high level of expression of the four most important death receptors, Fas, TNF-receptor 1 (TNF-R1), TRAIL receptor 1, and TRAIL receptor 2. This enhanced death receptor expression is likely the result of evolutionary pressure to support the vital need for the liver to eliminate virus-infected and/or mutated cells, which is achieved via engagement of death receptor–mediated apoptotic pathways by death ligand-expressing immune cells. Indeed, both TRAILand Fas-mediated apoptosis are essential components of cancer immunosurveillance by T cells and natural killer cells in the liver. However, this high sensitivity to death receptor–mediated cytotoxic pathways can also expose the liver to massive tissue damage if the receptors are excessively or chronically activated. For example, liver damage during viral hepatitis, a disease generally caused by infection with hepatitis B or hepatitis C virus, largely results from death receptor–mediated apoptosis associated with the host immune response to the viral antigen and not from a direct cytotoxic effect of the virus. Similarly, death receptor–mediated apoptosis has been involved in the pathogenesis of several other liver diseases, including alcoholic hepatitis, cholestatic liver disease, Wilson’s disease, and nonalcoholic steatohepatitis (NASH). For example, the liver of patients with NASH shows elevated Fas expression and increased sensitivity to Fasmediated apoptosis. Moreover, hepatocyte growth factor (HGF) receptor, Met, usually associates directly with Fas in normal liver, thus preventing its activation by FasL. Recent studies showed that in steatotic livers, this association is disrupted, whereas HGF and FasL expression is elevated, resulting in increased hepatocyte apoptosis and liver injury. The same studies also demonstrated that the use of a synthetic peptide mimicking the Fasbinding motif on Met effectively protects the liver from Fas-mediated apoptosis and liver damage, demonstrating that Fas apoptosis is a key event in the pathogenesis

Increased Fas expression

NHigh susceptibility to Fas Extensive liver damage

APOPTOSIS

Hepatocyte

Figure 21-3. Schematic representation of di erent susceptibility of hepatocytes to Fas-mediated apoptosis in normal and steatotic livers. In normal hepatocytes, the HGF receptor Met associates with Fas on the plasma membrane, preventing its activation by FasL. In hepatocytes from steatotic livers, Fas is over-expressed and no longer associates with Met; in addition, FasL and HGF are also over-expressed, resulting in increased hepatocyte apoptosis and liver injury.

of NASH (Figure 21-3). Death receptor engagement, however, does not always result in cell death. In particular, nonapoptotic signals can be activated by death ligands in cells that have acquired resistance to apoptosis through mutations of crucial components of the apoptotic machinery, including, but not limited to, loss-of- function mutations in the death receptor gene, upregulation of antiapoptotic Bcl-2 family members or cFLIP, or downregulation of proapoptotic Bcl-2 proteins. In these conditions, death receptors have been shown to promote oncogenic features, such as tumor proliferation, metastasis, and invasion via activation of NF-κB– and MAPK-regulated survival pathways. Therefore, an insult to the liver (i.e., viral infection, alcohol intake) generates an initial apoptotic response mediated by the immune cells that aims to eliminate the damaged or infected cells. If the exposure to the toxic agent persists, excessive apoptosis and chronic inflammation may favor the accumulation of genetic mutations, in particular mutations of tumor suppressor genes (i.e., p53) and genes involved in apoptosis signaling, and promote development of hepatocellular carcinoma. If the cancer cells have acquired resistance to death receptor–mediated apoptosis, engagement of death receptors could actually result in generation of a more aggressive phenotype. This observation has great clinical relevance in particular for

TRAIL, the use of which in cancer therapy is currently under evaluation.

6. PANCREAS

The pancreas is divided into lobules by septae of connective tissue. Each lobule is composed largely of grapelike cluster of exocrine cells called acini, which secrete the pancreatic juice containing digestive enzymes. This is collected into a tree-like series of ducts that run within the organ and is finally delivered into the duodenum through the main pancreatic duct. Scattered within the pancreatic exocrine tissue are clusters of cells called islets of Langerhans, which represent the endocrine component of the pancreas and produce several important hormones, including insulin, glucagon, and somatostatin. For the purpose of this chapter, we focus only on the exocrine component of the pancreas as a functional part of the GI system.

Apoptosis is involved in both development and progression of several pancreatic diseases, including acute and chronic pancreatitis and pancreatic ductal adenocarcinoma (PDAC). Acute pancreatitis is a disease associated with variable severity from mild, self-limited attacks, to severe, highly morbid, and frequently lethal attacks. The first acinar cell response to the injury seems to be the main factor determining the disease severity. In particular, extensive apoptosis of acinar cells is associated with mild acute pancreatitis, whereas severe acute pancreatitis involves extensive acinar cell necrosis, but very little acinar cell apoptosis. Little is known about the mechanism of apoptosis in the pancreatic acinar cells, but evidence of mitochondrial dysfunction points to the involvement of the intrinsic pathway of apoptosis. The massive release of cellular content occurring during necrotic cell death is likely responsible for recruitment of inflammatory cells and generation of inflammatory mediators, which negatively affect the course of the disease. NF-κB, which plays a critical role in the inflammatory response by regulating transcription of inflammatory cytokines such as TNF-α, IL-1β, and IL-6, is activated early in pancreatic acinar cells in models of experimental acute pancreatitis. Among the first response to NF-κB activation in acinar cells is the induction of the acute phase protein pancreatitis-associated protein-1 (PAP1), which reduces the extent of necrosis and infiltration of immune cells. Recent studies have shown that functional inactivation of NF-κB in acinar cells increases the susceptibility of these cells to inflammation-induced cell death and results in severe necrotizing pancreatitis, supporting a protective and organ-specific function of NF-κB during acute pancreatitis.

Chronic pancreatitis is characterized by progressive loss of acinar parenchyma and aggressive fibroinflammatory reactions, ultimately leading to irreversible organ destruction. Chronic inflammation induces neoexpression of death receptors (especially Fas and TRAIL receptors) in pancreatic acini, which become susceptible to apoptosis triggered by death ligands expressed on lymphocyte and released by pancreatic stellate cells (PSCs). In contrast, islets remain relatively intact because of failure to express functional death receptors and activation of NF-κB–induced antiapoptotic factors. Activation of PSCs plays a crucial role in pancreatic fibrogenesis during chronic pancreatitis; several inflammatory mediators, including transforming growth factor-beta (TGF-β), platelet-derived growth factor (PDGF), IL-1, IL-6, and TNF-α, stimulate the phenotypic changes from quiescent to active form.

Chronic inflammation is also a well-recognized risk factor for the development of PDAC. In PDAC, a variety of growth factors and growth factor receptors are expressed at increased levels. For example, over-expression of both EGF receptor and its ligands EGF and TGF-α is often observed in PDAC and is associated with enhanced tumor aggressiveness and shorter survival after tumor resection. In addition, PDAC often exhibits alterations in growth inhibitory pathways, such as Smad4 mutations and Smad6 and Smad7 over-expression, and evades apoptosis through mutations of tumor suppressor genes such as p53 and p16 and aberrant expression of apoptosis-regulating genes, including members of the Bcl-2 family. These alterations combined give pancreatic cancer a distinct growth advantage that clinically results in rapid tumor progression and poor survival prognosis. Also, pancreatic carcinoma cells have developed different mechanisms to evade the host immune surveillance. One is the expression of nonfunctional receptors (decoy receptors) and antiapoptotic members of the Bcl-2 family, such as Bcl-2 and Bcl-XL. Another is the expression of apoptosis-inducing ligands, such as FasL and TRAIL, which are able to trigger apoptosis of immune cells, creating areas of immune privilege. Similarly to what is observed for hepatocarcinomas, successful treatment of malignant tumors by recombinant TRAIL might be possible in some cases, but not in all pancreatic tumors, because of their differential resistance to TRAIL-induced cell death.

7. SUMMARY AND CONCLUDING REMARKS

The GI tract is characterized by a very dynamic epithelium that undergoes constant renewal to ensure

replacement of cells that have been damaged by toxins and/or microorganisms entering the body with food. Apoptosis plays a fundamental role in this process by counterbalancing the number of cells generated by division and differentiation of precursor stem cells. Overactivation of apoptosis can lead to significant tissue damage, whereas inhibition of apoptosis can promote proliferation and oncogenic transformation of cells. Cells bearing chromosomal alterations or mutated DNA are generally eliminated by apoptosis, mainly via activation of p53, thus preventing malignant transformation. Indeed, immortalization, a process characterized by unlimited potential to divide and resistance to cell death, is an important event in tumorigenesis, which probably occurs early in the neoplastic process. Consistently, many infectious and immune-mediated GI diseases, such as gastritis, viral hepatitis, and IBD, are initially associated with increased apoptosis and tissue damage, which generates inflammatory, fibrogenetic, and immune reactions accompanied by alterations of cellular growth and death, often resulting in cancer of the organ. Chronic inflammation, in particular, is a known predisposing factor to the development of cancer in the GI tract. Thus therapeutic approaches aiming to modulate apoptosis have the potential to be an effective tool for treating GI diseases.

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